The present invention relates to a projection lens for making an extended projection of an image from a video generating source using a CRT or other projection tube on a screen and displaying the extended image on the screen and a projection type image display apparatus using the same, and more particularly to a projection lens apparatus preferable for obtaining a high-contrast and bright image superior in a focusing performance with very little image distortion and a rear projection type image display apparatus using the same.
In recent years, a tendency of horizontally widening of a screen of a television set as a home image display apparatus has brought with it a large-sized screen thereof. There are two types of this home image display apparatus; a direct-view type using a cathode-ray tube (CRT), and a rear projection type for making an extended projection of an image from a video generating source, a small projection tube (CRT) having a size of 5 to 7 inches or so, on a screen from the rear side using a projection lens apparatus. From a viewpoint of compactness, weight and cost of the set, a rear projection type image display apparatus is in the mainstream as one having a screen size in excess of 37 inches.
As a projection lens apparatus for use in a rear projection type image display apparatus, there are known projection lens apparatuses described in JP-A-7-159688 (hereinafter, referred to as a first conventional art) and in JP-A-9-159914 (hereinafter, referred to as a second conventional art), for example. In the first conventional art, there is disclosed a lens of construction 5-groups 5-elements having a spherical glass lens of a low dispersion and a high refractive index and four aspherical lenses, as a power lens having the strongest positive refracting power included in the third lens group 3. In the second conventional art, there is disclosed a lens of construction 6-groups 6-elements having a spherical glass lens of a high dispersion and a low refractive index and five aspherical lenses, as a power lens.
The projection lens apparatus is required to have a short focal length in order to realize a compact set, to have a bright screen peripheral portion and a favorable focusing performance, and to be manufactured at low cost. In order to realize the low cost, it is the most effective to decrease the number of lenses in a lens construction to the minimum and to use an inexpensive optical glass lens as a power lens. In general, the higher the refractive index is and the lower the dispersion is, the more expensive the optical glass is.
The optical glass used for the power lens of the first conventional art is a low-dispersion glass having high refracting power, SK16. Regarding a price of this optical glass, the price of SK16 is twice or more times (2.1) that of SK5 as a reference (1.0), which is a typical optical glass for use in a projection lens apparatus. Therefore, in the first conventional art, the lens construction of 5-elements is applied to reduce the cost.
If an inexpensive spherical glass lens of a high dispersion and a low refractive index is used as a power lens for further cost reduction, however, not only it becomes harder to obtain desired refracting power, but a generated aberration amount also increases. Furthermore, since an aspherical plastic lens having a role of correcting aberrations has 10 or lower degree of an aspherical coefficient, an aberration correcting capability is insufficient due to restrictions of a permissible lens shape and the number of aspherical lens elements. As a result, while it is possible to realize a low cost, it becomes hard to favorably correct aberrations. It is hard to realize a low cost by using an expensive spherical glass lens of a low dispersion and a high refractive index.
A generated amount of an image distortion such as a distortion largely depends upon a positional relationship between a power lens (a glass lens) and an entrance pupil. The distortion is a phenomenon that an original image from a video generating source is projected in a form of a distorted image on a screen due to a difference between a magnifying power on an optical axis (paraxial) and a magnifying power in a peripheral portion. If the entrance pupil is located in the screen side of the power lens, a peripheral magnifying power is higher than the paraxial magnifying power and therefore it causes a pincushion distortion, while if the entrance pupil is located in the video generating source side of the power lens, the peripheral magnifying power is lower than the paraxial magnifying power and therefore it causes a barrel distortion. In other words, the distortion is caused by an increase of a difference between the peripheral magnifying power and the paraxial magnifying power since the chief ray passing through the center of the entrance pupil from each object point on the video generating source passes over a location apart from the optical axis of the power lens.
In the first conventional art, there is arranged a second group lens, which corrects a spherical aberration and a coma aberration, having weak negative refracting power in the screen side of the power lens. Therefore, if it is attempted to realize a further wide field angle (short projection distance), the position of the entrance pupil of the entire lens system shifts from the center of the third group lens toward the screen. This results in an increase of the distortion. Due to the aberration correcting capability of the aspherical plastic lens as set forth in the above, it is hard to correct the distortion favorably.
In the second conventional art, the cost reduction is possible to some extent due to a use of an inexpensive spherical glass lens having a high dispersion and a low refractive index as a power lens. The second conventional art, however, requires further aspherical lens for an aberration correction (one more element than the first conventional art) for favorably correcting the aberration increased by using the spherical glass lens having a high dispersion and a low refractive index. Therefore, it results in unsuccessful drastic cost reduction.
In addition, the projection lens apparatus is required to improve a contrast. Generally, importance is attached to an aberration correcting capability of a projection lens as a lens performance of a projection lens apparatus. As an image quality in a rear projection type image display apparatus, however, an improvement of a contrast indicating a white-and-black ratio of an image becomes an important element for determining whether or not the lens performance is good. To improve the contrast, there is a need for arranging respective lens groups spaced as far as possible from each other to prevent a reflected light (unnecessary light) in each lens group in the projection lens apparatus from returning to the original image. This results in an increase of aberrations in almost all cases, thereby requiring a higher correcting capability for correcting the aberration favorably. In the arrangements of the first and second conventional arts, it is hard to further improve the correcting capability, and therefore it is also hard to improve the contrast.
Generally, to satisfy conditions of a desired magnifying power and performance in such a case that a projection lens apparatus is used for a rear projection type image display apparatus having a wide angle of field, there is a need for arranging a glass lens having the strongest power among the lenses of the projection lens apparatus in the side of the video generating source. Accordingly, with a change of a relative position of the entrance pupil and the power lens (glass lens), a position of the chief ray from each object point on the video generating source changes and the image distortion and an astigmatism increases, by which it becomes hard to correct the aberration.
Furthermore, an arrangement of a power lens in the side of the video generating source extends a width of a luminous flux incident on a plastic lens having a role of correcting aberrations arranged in the screen side of the power lens. Therefore, to achieve brightness equivalent to an initial performance, there is a need for an enlargement of an aperture of this plastic lens for correcting the aberration and the enlargement of the lens aperture becomes a factor in a difficulty of the cost reduction.
In view of the above problems of the conventional arts, the present invention has been provided. It is an object of the present invention to provide a projection lens apparatus capable of reducing a cost in a wide angle of field and of correcting an aberration favorably, and a rear projection type image display apparatus using the same.
It is another object of the present invention to provide a projection lens apparatus which enables an improvement of a contrast, and a rear projection type image display apparatus using the same.
In other words, in the present invention, various devices are made for a single piece of projection lens and for an application of it to an apparatus. Specifically, in accordance with a first aspect, a second aspect, and sixth aspect of the present invention, the present invention is characterized by an arrangement in which an entrance pupil of a projection lens apparatus is located between a light incident surface and a light emitting surface of a power lens (included in the third lens group 3) having the strongest positive refracting power among a plurality of lens elements in the projection lens apparatus. This causes the chief ray from each object point on the video generating source to pass in the vicinity of the optical axis on the incident and emitting surfaces of the glass lens, thereby lowering a frequency of an occurrence of a distortion and astigmatism. Then, even if a construction of 5-groups 5-elements is applied by using an inexpensive glass material having an Abbe number vd of 60 or higher and a refractive index (nd) of 1.600 or lower for a power lens, for example, it is possible to suppress an occurrence of a distortion and astigmatism to a low level so as to secure a favorable correcting capability.
As set forth hereinabove, the present invention has a lens construction enabling an occurrence of an image distortion to be fully suppressed. Therefore, even if an inexpensive spherical glass lens having a high dispersion and a low refractive index is used as a power lens, for example, it is possible to fully correct an aberration by using four aspherical lenses, for example. Therefore, in this condition, desired brightness and focusing performance can be achieved. In other words, according to the present invention, it is possible to realize a low-cost projection lens apparatus having favorable image brightness and focusing performance, and a projection type image display apparatus using the same.
For insufficient refracting power caused by a use of an inexpensive optical glass having a low refractive index for a power lens, a desired refracting power may be obtained by distributing it to the second lens group 2 including an aspherical plastic lens for correcting a spherical aberration and a coma aberration arranged in the screen side of the glass lens.
An increase of the refracting power of the aspherical plastic lens accelerates deterioration of a performance in response to a temperature change or moisture penetration. To prevent it, preferably a curvature radius is increased (166 mm or higher) on one of the surfaces of the glass lens, which affects the performance most significantly among a plurality of lens elements. This reduces sensitivity to eccentricity or inclination caused by deterioration of an assembly precision of a lens tube, which may be made in response to a temperature change or moisture penetration, so as to prevent deterioration of a performance in response to the temperature change or moisture penetration.
A use of the inexpensive optical glass having a low refractive index for the power lens causes the refracting power to be insufficient, by which the spherical aberration increases. To cope with this, preferably a lens in an aspherical form represented by 14 or higher degree of an aspherical coefficient is used as an aspherical plastic lens having a role of correcting aberrations in the projection lens apparatus.
As set forth in the above, by using many aspherical shapes each having an aspherical amount and a complicated form, it becomes possible to achieve a capability of correcting an aberration in a ray which passes through a peripheral portion of the entrance pupil from each object point on the video generating source.
A decrease of a contrast in the projection lens apparatus is caused by a reflection of a reflected light generated on an air-side interface (light emitting surface) of a meniscus lens having a concave surface toward the screen, arranged in a position closest to the video generating source. This reflected light reduces the contrast by returning to a low luminance portion of an original image which appears on a display surface of the video generating source. Therefore, in the present invention, preferably the apparatus has a distance between the light emitting surface and an incident surface of the phosphor surface glass of the meniscus lens so as to be as long as possible. This does not decrease the amount of the entire reflected light generated on the air-side interface (light emitting surface) of the meniscus lens and returning to the video generating source, but it elongates an optical path of the reflected light up to the video generating source, by which the reflected light is spread out, thereby decreasing an intensity per area. It improves the contrast.
Furthermore, preferably there can be a wavelength selective filter for absorbing wavelengths other than a dominant wavelength of the original image in at least one of the meniscus lens or lens and coolant for cooling the video generating source put in contact with the meniscus lens. This reduces the reflected light effectively and suppresses generation of a chromatic aberration.
Finally, the problem of realizing a wide angle of field of the projection lens can be resolved by increasing a power distribution of the third lens group 3 of the projection lens. This not only enables an achievement of a desired magnifying power, but also minimizes a spread of a width of a luminous flux incident on the first group lens and the second group lens having a role of correcting aberrations arranged in the screen side of the power lens. This makes it possible to cope with a wide angle of field without increasing an aperture of the plastic lens for correcting aberrations.
In addition, by increasing a power distribution of the third lens group, a lateral aberration in the meridional direction of a ray in a periphery can also be controlled due to a high positive refractive index in a peripheral portion of the emitting surface of the third lens group and a negative refracting power and an aspherical shape in a peripheral portion of the emitting surface of the first lens group. Therefore, it is possible to cope with a wide angle of field with keeping the aberration correcting capability favorably.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.